Window quotes are full of jargon, and installers do not always stop to explain it. This glossary defines the terms you are most likely to meet when buying double glazing in 2026, from the frame materials and window styles to the energy ratings that decide whether a window meets Building Regulations. Each entry is short and factual. For the figures behind the words, the cost per window guide sets out 2026 prices, and you can build your own estimate in the uPVC window calculator.
Terms A to Z
A-rated glazing
A window with a Window Energy Rating (WER) of band A. A-rated double glazing is the common standard for new replacement windows in 2026.
Argon gas
An inert gas sealed between the panes of a double or triple glazed unit. It conducts heat less readily than air, lowering the U-value.
Bay window
A window that projects outward from the wall in three or more panes, creating a recess inside. More complex and costly to replace than a flat run of windows.
Casement window
A window hinged at the side or top that opens outward. The most common and affordable style in UK homes.
Double glazing
A sealed unit of two glass panes with a gas-filled gap between them, which cuts heat loss and noise compared with single glazing.
FENSA
The Fenestration Self-Assessment Scheme. A FENSA certificate proves replacement windows meet Building Regulations without separate council sign-off. See our FENSA guide for more.
Low-E glass
Glass with a microscopically thin metal-oxide coating that reflects heat back into the room while letting light through. It cuts the U-value by around 40%.
Sash window
A window with one or two panels (sashes) that slide vertically. Traditional in Victorian and Georgian homes.
Tilt and turn
A window that can tilt inward from the top for ventilation or swing fully open like a door, useful on flats and upper floors.
Triple glazing
A sealed unit of three glass panes with two gas-filled gaps. It improves insulation and noise reduction but adds roughly 20 to 30% to the cost.
U-value
A measure of how much heat passes through a window, in W/m²K. Lower is better. Replacement windows must reach 1.4 W/m²K or lower under Part L. See our full U-values guide for more.
uPVC
Unplasticised polyvinyl chloride, the most common and affordable window frame material. Low maintenance with a 20 to 25 year life.
Warm-edge spacer
The strip that separates the glass panes around the edge of a sealed unit. A warm-edge (low-conductivity) spacer cuts edge heat loss and condensation.
Window Energy Rating (WER)
A BFRC label rating a window from A++ (best) to G, combining U-value, solar gain and air leakage. Part L (since 15 June 2023) accepts band B or better as one route to compliance for replacement windows, the alternative being a U-value of 1.4 W/m²K or lower.